Upgrading the existing metallurgy in a fired heater asset is a logical strategy for mitigating corrosion rates. However, metallurgical tube upgrades alone do not immunise an asset against corrosion, or eliminate the need for regular corrosion monitoring.

If unexpected wall loss is detected following a metallurgical tube upgrade, it is critical to assess the damage and take steps to prevent future asset failure. Particularly in cases of an unknown damage mechanism, in-depth inspection and assessment can accurately determine the condition of an asset. Ultrasonic in-line “smart pig” inspection technology can detect and quantify a multitude of defects within furnace heater coils, aiding operators in identifying potential and existing problems and preventing unplanned shutdowns.

Background

When a refinery experienced unexpected metal loss in the radiant section of a coker furnace, an ultrasonic in-line inspection and metallurgical assessment was performed to assess the condition of the asset. Prior to the in-line inspection, manual spot UT inspection results revealed wall thinning on the radiant tubes. This wall loss was not anticipated since the heater tubes consisted of high grade stainless steel, and had been running within its normal operating conditions.

In-Line Inspection

During the previous metallurgical upgrade, flat back return bends were installed in the coker furnace, which posed a navigational challenge to conventional in-line inspection technology. In order to successfully navigate this type of furnace configuration, a unique navigation technique was used to overcome this challenge. In order to demonstrate this technique, a test inspection was conducted at the ILI provider’s testing facility, using a spare return bend from the heater. This test run included both the cleaning and inspection tools, verifying the tool’s navigational capabilities and ensuring accurate data collection.

The successful in-line inspection was completed on the furnace in 2016. Because it was the first complete in-line inspection performed on this particular asset, there was no previous in-line inspection data on which to make a detailed comparison. However, it was found that general wall thinning was occurring throughout the unit, with the most extensive damage occurring in the radiant section of the furnace.

Metallurgical Engineering Assessment

The most extensively damaged section of piping had been removed prior to the inspection in order to perform a metallurgical assessment.

Figure 1. Section of tubing removed from coker furnace.

After reviewing the operating history of the unit compared to its material properties, it was determined that the radiant tube metal loss was higher than expected, based upon the installed tube material and sulfur content of the feed. No creep damage in the form of diametrical growth, bulging or swelling was detected in the radiant or convection tubes.

A follow-up inspection of this unit was conducted six months later, to monitor for any changes occurring in the unit. This inspection found that there were no significant changes in the radiant section’s tube thickness, verifying that the integrity of the asset was not degrading at an unusual rate under the current feed stock and operating conditions. By utilizing in-line inspection, advanced engineering and metallurgical testing, the owner-operator was able to achieve a comprehensive look at the unit’s long-term condition from a number of assessment perspectives.

The Importance of Verification

Routine in-line inspection is a prudent strategy for managing the long-term condition of furnace units, providing operators with a complete and quantitative wall thickness map. Although the owner-operator took the proper best practice measures by upgrading the metallurgy of the tubing, it was found that unexpected damage was continuing to occur. By customizing an inspection methodology and testing it for the specific challenging heater configuration in question, an in-line inspection and more complete heater cleaning strategy was made possible. In combining this inspection data with metallurgical testing and engineering assessments, the refinery gained a better understanding of not only the unit in its current state, but also received the confidence to facilitate effective future maintenance planning and significant cost management.